We have evaluated the cross-sectional area of the maxillary sinus of 37 Korean adult(Male 17 and Female 20) having a normal sinus state by frontal section using C-T scan and Water's projection. A frontal section of 3 cm from nasion was used. We have produced the area showing on the film by two method, direct and indirect. The direct method was directly calculated of plotted area and indirect method was to use an approximated cross-sectional configuration of the maxillary sinus. The calculation of the area was used by paper divided into squares of which the unit area is 1 mm. Magnification power of both was ×1.15 and was not corrected. The results were analyzed as to difference between the sides and gender, and the linear correlation between two methods of evaluation for the cross-sectional area obtained : 1) In the area resulting from frontal section using C-T scan. (1) In the area calculated from direct method, male average(147.2 mm²) and female average(126.3mm²) were showed. (2) In the area calculated from indirect method, male average(155.0mm²) and female average(133.9mm²) were showed. (3) Statistically no significant difference was seen between the right and left side. (4) The results for male were larger than those of female with statistical significance. (5) The linear correlation between the direct and indirect method is very high(R=0.867). 2) In the area resulting from water's projection. (1) In the area calculated from direct method, male average(154.9mm²) and female average(131.8mm²). (2) In the area calculated from indirect method, male average(162.7mm²) and female average(140.3mm²). (3) Stastistically no significant difference was seen between the right and left side. (4) The result for male were larger than those of female with statistical significance. (5) The linear correlation between the direct and indirect method is relatively high(R=0.755). 3) Indirect calculating method is easy method when it is difficulty in calculating of the cross-sectional area of the maxillary sinus cavity with morphological changes.